X-ray CT scanner

ABSTRACT

An X-ray CT scanner includes an X-ray tube which irradiates an object, a variable X-ray limiting device which limits the X-ray beam thickness, an X-ray detector which detects X rays transmitted through the object and has a plurality of detecting elements arrayed in a matrix manner, a storing unit which stores a plurality of calibration data files corresponding to a plurality of beam thicknesses, a correcting unit which corrects an output from the X-ray detector on the basis of at least one calibration data file read out from the storing unit, a reconstructing unit which reconstructs image data concerning the object on the basis of an output from the correcting unit, and a control unit which controls the variable X-ray limiting device to change the beam thickness of the X rays, independently of the beam thicknesses to which the calibration data files stored correspond.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-325920, filed Oct.25, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an X-ray CT scanner having acorrecting function.

[0004] 2. Description of the Related Art

[0005] An X-ray CT scanner is an apparatus which generates tomogram databy reconstructing, by using a computer, projection data obtained byirradiating an object to be examined with X-rays from the circumferenceof the object. These X-ray CT scanners are classified into the followingthree types in accordance with differences between the forms of X-raybeams.

[0006] The first one is a “fan-beam X-ray CT scanner” which radiates afan-shaped X-ray beam from an X-ray tube. This fan-beam X-ray CT scanneracquires projection data by using an X-ray detector obtained byarranging about, e.g., 1,000 channels of detecting elements in a line.Projection data acquiring operation is repeated about 1,000 times whilethe X-ray tube rotates around an object to be examined. This fan-beamX-ray CT scanner is also called a “single-slice CT scanner” because dataconcerning a single slice are acquired.

[0007] The second one is a so-called “multi-slice X-ray CT scanner” inwhich several X-ray detectors each obtained by arranging about 1,000channels of detecting elements in a line are juxtaposed in a slicedirection. A slightly thick X-ray beam is radiated in accordance withthe width of these juxtaposed detectors. This multi-slice X-ray CTscanner is so called because data of several slices can besimultaneously acquired.

[0008] The third one is a so-called “cone-beam X-ray CT scanner” inwhich a plurality of detecting elements each composed of a combinationof, e.g., a scintillator and a photodiode are two-dimensionally arrayed.A conical or pyramidal X-ray beam is radiated in accordance with thewidth of these detecting elements in a slice direction. This cone-beamX-ray CT scanner is also called a volume X-ray CT scanner because volumedata can be acquired at once.

[0009] The research of a cone-beam X-ray CT scanner has been advancedprimarily on a system using an image intensifier (I.I.) as an X-raydetector since late 1980 s. For example, in “Volume CT ofanthropomorphic using a radiation therapy simulator” (Michael D. Silver,Yasuo Saito et al.; SPIE 1651 197-211 (1992)), the results of scan ofchest phantoms in an experimental system combining a turntable and anI.I. are discussed. Some cone-beam X-ray CT scanners are beginning to beput into practical use as apparatuses for obtaining the shapes ofhigh-contrast objects such as bones and blood vessels in angiography.

[0010] As described above, a cone-beam X-ray CT scanner has a widerdivergent angle of an X-ray beam in a slice direction than in the othertwo types. In other words, the X-ray beam is thick on the rotationcentral axis. Since this increases the number of paths through whichscattered rays reach detecting elements, the scattered ray amountincreases. Scattered rays cause abuses, e.g., deteriorate the imagecontrast. This scattered ray increasing mechanism means that thescattered ray amount varies in accordance with a change in the beamthickness.

[0011] An X-ray CT scanner usually performs sensitivity correction inorder to equalize the sensitivities of detecting elements. For thispurpose, calibration data files (calibration data) for sensitivitycorrection are acquired by using a phantom (pseudo model). Sincescattered rays change in accordance with the beam thickness as describedabove, these calibration data files must also be selectively used inaccordance with the beam thickness.

[0012] This paradoxically means that the degree of freedom of beamthickness adjustment is limited by the types of calibration data filesthat the apparatus has.

[0013] Assume, for example, that a calibration data file acquired by abeam thickness X1 and a calibration data file acquired by a beamthickness X2 (>X1) are prepared. In this case, no correspondingcalibration data files are prepared for beam thicknesses other than X1and X2. Therefore, no such beam thicknesses can be set except when theinclusion of a scattered ray error is permitted.

BRIEF SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide an X-ray CTscanner capable of extending the degree of freedom of setting of anX-ray beam thickness.

[0015] According to a certain aspect of the present invention, an X-rayCT scanner comprises an X-ray tube which irradiates an object to beexamined with X rays, a variable X-ray limiting device which limits thebeam thickness of the X rays, an X-ray detector which detects X raystransmitted through the object and has a plurality of detecting elementsarrayed in a matrix manner, a storing unit which stores a plurality ofcalibration data files corresponding to a plurality of beam thicknesses,a correcting unit which corrects an output from the X-ray detector onthe basis of at least one calibration data file read out from thestoring unit, a reconstructing unit which reconstructs image dataconcerning the object on the basis of an output from the correctingunit, and a control unit which controls the variable X-ray limitingdevice to change the beam thickness of the X rays, independently of theplurality of beam thicknesses to which the plurality of calibration datafiles stored correspond.

[0016] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0017] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of theinvention and, together with the generation description given above andthe detailed description of the embodiments given below, serve toexplain the principles of the invention.

[0018]FIG. 1 is a schematic view showing the arrangement of an X-ray CTscanner according to an embodiment;

[0019]FIG. 2 is a view conceptually showing the flow of data in theX-ray CT scanner shown in FIG. 1;

[0020]FIG. 3 is a flow chart showing the flow of a calibration data fileacquisition process in the X-ray CT scanner shown in FIG. 1;

[0021]FIG. 4 is a view showing six different beam thicknesses to which aplurality of calibration data files stored in a storing unit shown inFIG. 1 correspond;

[0022]FIG. 5 is a view showing the relationship between the sixdifferent beam thicknesses shown in FIG. 4 and an X-ray detector;

[0023]FIG. 6 is a view showing six calibration data files correspondingto the six different beam thicknesses shown in FIG. 4;

[0024]FIGS. 7A and 7B are views showing the relationship between thebeam thickness and the scattered ray amount in this embodiment;

[0025]FIGS. 8A and 8B are views showing the relationship between thesize of an object to be examined and the scattered ray amount;

[0026]FIG. 9 is a flow chart showing the flow of an object scanningprocess in this embodiment;

[0027]FIGS. 10A and 10B are views showing a beam thickness input windowin a screen;

[0028]FIG. 11 is a view showing geometry when an object to be examinedis scanned in this embodiment;

[0029]FIG. 12 is a view showing the first example of a method ofgenerating a calibration data file for use corresponding to a set beamthickness in this embodiment;

[0030]FIG. 13 is a supplementary view of FIG. 12;

[0031]FIG. 14 is a view showing the second example of the method ofgenerating a calibration data file for use corresponding to a set beamthickness in this embodiment;

[0032]FIG. 15 is a view showing the third example of the method ofgenerating a calibration data file for use corresponding to a set beamthickness in this embodiment;

[0033]FIG. 16 is a view showing the fourth example of the method ofgenerating a calibration data file for use corresponding to a set beamthickness in this embodiment;

[0034]FIG. 17 is a view showing the fifth example of the method ofgenerating a calibration data file for use corresponding to a set beamthickness in this embodiment;

[0035]FIG. 18 is a view showing a method of determining a calibrationdata file concerning a beam thickness closest to a set beam thickness inthis embodiment;

[0036]FIG. 19 is a view showing a system configuration for practicing ascattered ray correction process, disclosed in Jpn. Pat. No. 1631264;

[0037]FIG. 20 is a view showing a system configuration for practicing ascattered ray correction process, disclosed in Jpn. Pat. Appln. KOKAIPublication No. 11-89827;

[0038]FIGS. 21A and 21B are graphs showing the forms of X-ray componentsobtained by an X-ray detector in the system configuration shown in FIG.20; and

[0039]FIG. 22 is a view for explaining a status in which it is moreadvantageous not to practice a scattered ray correction process.

DETAILED DESCRIPTION OF THE INVENTION

[0040] An embodiment of the present invention will be described belowwith reference the accompanying drawing.

[0041]FIG. 1 is a schematic view showing the arrangement of an X-ray CTscanner according to this embodiment. Referring to FIG. 1, an X-ray CTscanner 1 includes a frame 11 and a console 12. The frame 11 has ahollow portion 11 a. Into this hollow portion 11 a, a patient P placedon a table top 11 b of a bed is inserted. An X-ray tube 111 and an X-raydetector 112 are arranged around the hollow portion 11 a. The X-ray tube111 and the X-ray detector 112 are mounted to oppose each other on arotary ring 11 c held to be rotatable around a rotation central axis RAperpendicular to the drawing surface. The X-ray tube 111 is connected toan X-ray generating unit 111 a including a high-voltage power supply.The X-ray detector 112 includes a plurality of detecting elements eachcomposed of, e.g., a scintillator and a photodiode. These detectingelements are arrayed in a matrix manner in two directions, i.e., adirection parallel to the rotation central axis RA, and a directionsubstantially perpendicular to the rotation central axis RA. Note thatthe direction parallel to the rotation central axis RA will be referredto as a “slice direction” hereinafter, and the direction substantiallyperpendicular to the rotation central axis RA will be referred to as a“channel direction” hereinafter.

[0042] X-rays generated by the X-ray tube 111 irradiate the patient P asindicated by the broken lines in FIG. 1. X-rays transmitted through thepatient P are converted into electric signals by the detecting elementsof the X-ray detector 112 and acquired by a data acquisition system 122.

[0043] A variable X-ray limiting device (also called a collimator) 111 cis attached to an X-ray emission window of the X-ray tube 111. Thisvariable X-ray limiting device 111 c has a plurality of shielding platesto limit the beam thickness of X-rays generated from the X-ray tube 111in the slice direction. These shielding plates are so supported as to beindividually movable in the slice direction. The X-ray beam thicknesscan be varied by adjusting the spacings between these shielding plates.The collimator 111 c is typically a multi-leaf collimator. Thismulti-leaf collimator has a plurality of plate-like leaves constructingtwo leaf pairs. The multi-leaf collimator can freely limit the beamthickness of X-rays by moving these plate-like leaves independently ofeach other, such that the leaves come close to or move away from eachother in the longitudinal direction.

[0044] The console 12 includes, e.g., a central control unit 121, aninput device 127, and an image displaying unit 12D. The central controlunit 121 controls the frame 11, the bed, the table top, and the like.The input device 127 is used by an operator to access this centralcontrol unit 121. The image displaying unit 12D displays reconstructedCT images (e.g., an axial image, multi-planar reconstruction image (MPRimage), body surface image, and maximum intensity projection image (MIPimage)). Of these devices, the input device 127 can be a pointing devicesuch as a mouse or a track ball, and the image displaying unit 12D canbe a CRT or the like.

[0045] An operator inputs a command to the central control unit 121 viathe input device 127. In accordance with this input command, the centralcontrol unit 121 reconstructs tomogram data on the basis of an outputfrom the X-ray detector 112, and displays the data on the imagedisplaying unit 12D. More specifically, the tomogram or the like isreconstructed by the flow of data, or processing, conceptually shown inFIG. 2, in the data acquisition system 122, a preprocessing unit 123, amemory 124, a reconstructing unit 125, and a data processing unit 126shown in FIG. 1. The reconstructed image is displayed on the imagedisplaying unit 12D.

[0046] Referring to FIG. 2, the data acquisition system 122 receives aplurality of electric signals from a plurality of detecting elements ofthe X-ray detector 112. This data acquisition system 122 amplifies theseelectric signals and outputs the amplified electric signals as digitalsignals via an A/D converter. A digital signal before being subjected topreprocessing is called raw data.

[0047] The preprocessing unit 123 corrects the raw data from the dataacquisition system 122 on the basis of at least one calibration datafile read out from a plurality of calibration data files stored in astoring unit 12M. This correction process includes, e.g., referencecorrection, water correction, and sensitivity correction. The datacorrected by this preprocessing unit 123 is data immediately beforereconstruction and is called “projection data”.

[0048] The memory 124 stores this projection data. The reconstructingunit 125 receives the projection data from the memory 124. On the basisof this projection data, the reconstructing unit 125 reconstructs thedistribution (called volume data or voxel data) of X-ray absorptioncoefficients in a three-dimensional region extending in the slicedirection of the patient P, by using a three-dimensional imagereconstructing algorithm represented by, e.g., a method called aFeldkamp method. In the above description, the projection data is oncestored in the storing unit 124. In some cases, however, the projectiondata can be directly supplied from the preprocessing unit 123 to thereconstructing unit 125 without being stored in the memory 124.

[0049] The volume data is supplied to the data processing unit 126directly or after being once stored in the storing unit 12M. From thisvolume data, the data processing unit 126 generates image data fordisplay, such as a tomogram of a desired cross section, a transmissionimage of a desired direction, or a so-called three-dimensional imagecapable of two-dimensionally expressing a stereoscopic structure. Theimage displaying unit 12D displays this image data for display in grayscale or in color. The image data for display and the volume data arestored in the storing unit 12M typically implemented by a hard diskdrive.

[0050] Note that the arrangement of the X-ray CT scanner 1 shown in FIG.1 is merely an example. That is, in FIG. 1 the reconstructing unit 125and the like are configured as the console 12 separately from the frame11. However, the reconstructing unit 125 and the like can also beinstalled in the frame 11. It is also possible to install the dataacquisition unit 122 in the frame 11, and the preprocessing unit 123 andsubsequent units in the console 12. In this case, the transmission ofelectric signals from the former to the latter is performed using anon-contact data transmitting means (not shown).

[0051] A sensitivity correction process by the preprocessing unit 123will be described below with reference to flow charts shown in FIGS. 3and 9.

[0052]FIG. 3 shows a calibration data file acquisition process. Raw datais corrected on the basis of a calibration data file. This correction isthe process of equalizing the sensitivities of the detecting elements ofthe X-ray detector 112. By this correction, the CT value of water andthe CT value of air are standardized to “0” and “−1,000”, respectively.A calibration data file is generated from data acquired under the same“scan conditions” as actual examination by using a cylindrical modelfilled with water, i.e., a “water phantom”. The “scan conditions”include a field of view (FOV), an approximate diameter of the patient P,a tube voltage, a tube current, and the like.

[0053] The field of view FOV is a region as an object of reconstruction,and is formed into a columnar shape around the rotation central axis RA.The size of this field of view FOV is defined by its radius and length.Generally, the beam thickness of X-rays is so determined that the X-rayscover the entire field of view FOV. The X-ray beam thickness is definedas the thickness of an X-ray bundle on the rotation central axis RA.This X-ray beam thickness is determined by the size of the field of viewFOV. Conversely, when the X-ray beam thickness is determined, the sizeof the field of view FOV is determined accordingly. That is, the X-raybeam thickness and the size of the field of view FOV are parameterswhich define each other. In the following explanation, the term “X-raybeam thickness” is used, but this term can also be reread as the size ofthe field of view FOV.

[0054] In step S1 of FIG. 3, a phantom is placed between the X-ray tube111 and the X-ray detector 112, and this phantom is irradiated withX-rays limited to a specific beam thickness through the collimator 111c. In step S2, the X-rays transmitted through the phantom are detectedby the detector 112, and data (phantom data files) are acquired. Thisphantom data file acquisition is repeated every predetermined cyclewhile the X-ray tube 111 rotates around the phantom. Consequently, aplurality of phantom data files are acquired in one-to-onecorrespondence with a plurality of points discretely arranged at fixedintervals on the rotational orbit along which the X-ray tube 111 rotatesaround the phantom.

[0055] In step S3, the data processing unit 126 calculates a calibrationdata file from the acquired phantom data files. The method of thiscalculation can be an arbitrary one. For example, the average additionvalue is calculated for each channel from the acquired phantom datafiles. A set of these average addition values is a calibration datafile. Noise can be reduced by this addition average.

[0056] In step S4, the calibration data file thus calculated is storedin the storing unit 12M.

[0057] The routine from S1 to S4 is repeated until a plurality ofcalibration data files are acquired in one-to-one correspondence with aplurality of predetermined beam thicknesses (step S5).

[0058]FIG. 4 shows examples of a plurality of beam thicknessespreviously determined to acquire a plurality of calibration data files.FIG. 5 is a plan view showing the X-ray detector 112 viewed from a pointat which the X-ray tube 111 exists. FIG. 5 shows the relationshipbetween the range of the X-ray detector 112 within which effective datais detected, i.e., the use region of the X-ray detector 112, and thebeam thicknesses shown in FIG. 4.

[0059] As shown in FIGS. 4 and 5, in the slice direction of the patientP, a total of six different beam thicknesses, i.e., a maximum beamthickness “LL” determined by the use region of the X-ray detector 112,and subsequent beam thicknesses “L”, “M”, “μg” “SS”, and “SSS”, are setat substantially equal spacings. That is, six calibration data files areacquired in one-to-one correspondence with these six different beamthicknesses.

[0060] The X-ray detector 112 has a plurality of detecting elements 112a arranged in an m×n matrix manner in the two, slice and channeldirections. A center-to-center distance between the detecting elements112 a adjacent in the channel direction is, e.g., 1 mm, the distance isdefined as the distance on the rotation central axis RA. Acenter-to-center distance between the detecting elements 112 a adjacentin the slice direction is designed to be 1 mm, the same value, thedistance is defined as the distance on the rotation central axis RA.

[0061] The maximum beam thickness LL is given by n x 1 mm. In actualscanning, the beam thickness can be finely set in units of 1 mm from 1mm to n×1 mm. Six different calibration data files are acquired for beamthicknesses, in this embodiment the six different, discrete beamthicknesses, fewer than the settable beam thicknesses. It is of coursealso possible to acquire calibration data files for all the settablebeam thicknesses, but this is not practical. As is well known, thesensitivity of the detecting element 112 a varies with time.Accordingly, calibration data files must be updated whenever the mainpower supply is turned on, or periodically. If the calibration data fileacquiring operation is repeated for all the beam thicknesses wheneverupdate is executed, the time of this updating operation is significantlyincreased.

[0062]FIG. 6 shows the six different calibration data files obtained forthe six different beam thicknesses. As shown in FIG. 6, the larger thebeam thickness, the larger the value of calibration data. This is sobecause the amount of scattered rays increases as the region of thepatient P to be irradiated with X rays increases in size. The scatteredray amount increases because, as shown in FIGS. 7A and 7B, the largerthe beam thickness, the larger the number of incident paths of scatteredrays SX (indicated by the broken lines in FIGS. 7A and 7B). FIGS. 8A and8B schematically illustrate the relationship between the size (a radiusR and a length L) of the field of view FOV and the scattered ray amount.As depicted in FIGS. 8A and 8B, the scattered ray amount changes inaccordance with the size of the patient P. In FIGS. 8A and 8B, thesection of the X-ray detector 112 is a rectangle. However, this ismerely an example, and the section can also be a circular arc (FIG. 19).

[0063] As described above, the six calibration data files correspondingto the six different beam thicknesses are acquired.

[0064]FIG. 9 shows an actual scan procedure for the patient P. As instep T1, the beam thickness (or the size of FOV) is input in accordancewith the purpose of examination from the input device 127.

[0065] This beam thickness can be set to an arbitrary integral multipleof a unit length of 1 mm as the pitch of the detecting elements,regardless of the six different, discrete beam thicknesses to which thesix calibration data files correspond. Letting Xmax denote the beamthickness defined by the use region in the X-ray detector 112, a beamthickness Xt settable in this case can be expressed by 0<Xt≦Xmax. Thatis, the settable beam thickness Xt is substantially “arbitrary” withinthe range having Xmax as the upper limit, although the set pitchdescribed above is restricted.

[0066]FIG. 10A shows a beam thickness setting window. The beam thicknesssetting window is displayed with a patient information window and a scancondition table window in a screen. Two types of cursors CA and CB aredisplayed on a scanogram in order to set a beam thickness. The cursor CArepresents the centerline of the beam with respect to the slicedirection. The two cursors CB represent two ends of the beam withrespect to the slice direction. The operator manipulates a pointingdevice such as a mouse to move the cursor CA back and forth along theslice direction. This allows setting the beam center to a desiredposition. The operator manipulates the pointing device such as the mouseto one of the two cursors CB back and forth along the slice direction.The other cursor CB automatically moves upon movement of one cursor CBsuch that the distance between one cursor CB (one beam end) and thecursor CA (beam center) becomes equal to the distance between the othercursor CB (other beam end) and the cursor CA (beam center). This allowssetting the beam thickness to a desired thickness. The numerical valuesin start/end position cells of a beam thickness column are changedautomatically in accordance with the movements of the cursors CA and CB.Conversely the, positions of cursors CA and CB are changed automaticallyin accordance with the newal of the numerical values.

[0067]FIG. 10B shows another beam thickness setting window. The beamthickness setting window is displayed with a patient information windowand a scan condition table window in a screen. According to this method,two cursors CB representing the two ends of a beam with respect to theslice direction are used in order to set a beam thickness. The operatormanipulates the pointing device such as the mouse to move one cursor CBback and forth along the slice direction. The operator also manipulatesthe pointing device such as a mouse to move the other cursor CB back andforth along the slice direction. This allows setting the beam thicknessto a desired thickness and the beam center to a desired position.

[0068] Next, as in step T2, on the basis of at least one calibrationdata file corresponding to the set beam thickness, a “calibration datafile for use” to be used to correct raw data of the patient P isgenerated. The method of generating this calibration data file for usewill be described later. This “generation” process is performed by thecentral control unit 121 described earlier. In step T3, a slice apertureof the limiting device 111 c shown in FIG. 4 is set such that the setbeam thickness is obtained.

[0069] As shown in steps T4 to T7, the patient P is irradiated withX-rays to acquire raw data, and the acquired raw data is corrected bythe preprocessing unit 123. On the basis of projection data generated bythe correction, the reconstructing unit 125 reconstructs volume data.From this volume data, the data processing unit 126 generates image datafor display, such as a tomogram or a three-dimensional image. This imagedata for display is displayed on the image displaying unit 12D or storedin the storing unit 12M.

[0070] The correction process (step T5 in FIG. 9) that the preprocessingunit 123 performs for the raw data from the data acquisition system 122by using the calibration data file for use will be described in detailbelow.

[0071]FIG. 11 shows an example of geometry during scanning of thepatient P. The beam thickness is set as indicated by the thick line inFIG. 11. The set beam thickness does not match any of the six differentbeam thicknesses corresponding to the six calibration data files. Inthis example, the set beam thickness is intermediate between M and S. Ifno matching calibration data file exists, correction cannot be performedin conventional apparatuses. In the first place, beam thickness choicesare limited to beam thicknesses corresponding to calibration data files.In this embodiment, however, a calibration data file for use whichmatches an actually set beam thickness is generated from at least one ofthe six different calibration data files described above (step T2 inFIG. 9).

[0072] As the method of generating the calibration data file for use,five different methods from the first to fifth methods are provided. Thecentral control unit 121 can be equipped with one or all of these fivemethods. In the latter case, these five methods are selectively used inaccordance with a designation by an operator. The five different methodsof generating a calibration data file for use will be explained below inturn.

[0073] (First Method: Interpolation)

[0074] This first method obtains the aforementioned calibration datafile for use by interpolation from at least one calibration data fileselected in accordance with the set beam thickness from the sixdifferent existing calibration data files described above. As shown inFIG. 6, the values of these six different calibration data filesincrease as the beam thickness increases under the influence ofscattered rays. However, it is known that the relationship between thescattered ray amount and the beam thickness is substantially aproportional relationship. Therefore, linear interpolation using thebeam thickness as a parameter can be performed for each correspondingdetecting element of the X-ray detector 112.

[0075] For example, if the set beam thickness is between the M- andS-regions as shown in FIG. 11, two-point interpolation is performedusing calibration data files of the M- and S-regions, as indicated bythe alternate long and short dashed line in FIG. 12, for those detectingelements (in a region SI in FIG. 11) of the X-ray detector 112 which areinside the S-region. For detecting elements (in a region MS in FIG. 11)between the M- and S-regions, extrapolation is performed usingcalibration data files of the L- and M-regions, since there is nocalibration data file of the S-region. By these interpolating processes,a calibration data file AAl for use as shown in FIG. 12 is obtained.

[0076] In the present invention, as indicated by the alternate long andtwo short dashed line in FIG. 12, multi-point interpolation can also beperformed instead of the two-point interpolation. That is, for detectingelements in the region SI described above, interpolation can beperformed using three points on calibration data files of the L-, M-,and S-regions. For the region MS, interpolation can be performed usingthree points on calibration data files of the LL-, L-, and M-regions.Furthermore, in place of the above methods, interpolation can also beperformed using all the six different calibration data files for, e.g.,detecting elements in the region SI.

[0077] In the multi-point interpolation as described above, the numberof points to be used in interpolation can be properly determined fromthe relationship between the effect and the processing amount. Also, asdescribed above, FIG. 12 shows examples of two-point interpolation andthree-point interpolation. However, this simply means that the twomethods are illustrated in one figure for the sake of convenience ofexplanation. In practice, therefore, the calibration data file AA1 forall beam thicknesses is generally obtained by two-point interpolation orthree-point interpolation alone.

[0078] It is, however, exceptionally possible in some cases to usetwo-point interpolation and three-point or multi-point interpolation atthe same time. A case is, e.g., when it is desirable to acquire an imagehaving higher accuracy in a portion near the ordinate in FIG. 12, i.e.,in a central portion of the X-ray detector 112, than in other portions.In this case, it is possible to perform three-point interpolation nearthe ordinate and two-point interpolation in the other portions. Thepresent invention has no intention to positively exclude these forms.

[0079] Also, if the set beam thickness is present between the LL- andL-regions (a region L3) as shown in FIG. 13, it is impossible to obtaincalibration data files at a plurality of points necessary forinterpolation for obtaining a calibration data file AA2 for use. Thisbasically makes interpolation impossible to perform. The simplest methodin a case like this is to, e.g., use a calibration data file concerningthe LL-region directly as a calibration data file for use.

[0080] In the processing as described above, however, as shown in FIG.13, a calibration data file concerning the beam thickness inside theL-region and a calibration data file concerning the beam thickness inthe region L3 become discontinuous. This situation is not preferredbecause it may cause an artifact on the reconstructed image. In thismethod, therefore, the following method can be used instead of the aboveone.

[0081] That is, as shown in FIG. 13, it is possible to use informationcontained in an edge portion of a calibration data file pertaining tothe L-region, i.e., to use a differential coefficient, or to obtain anextrapolation point on the basis of an output value near the edge. Inthis manner, a calibration data file (to be referred to as an “extendedcalibration data file” hereinafter) EA which is extended to smoothlyconnect to the edge portion is formed. When two-point interpolation isperformed using this extended calibration data file EA and a calibrationdata file of the LL-region, a calibration data file AA′ for use havinghigher accuracy is obtained. In this method, no such discontinuousportion as mentioned above is produced.

[0082] This method of forming the extended calibration data file EA isgenerally applicable to a portion where the combination of calibrationdata files for use in interpolation changes, i.e., to an edge portion ofeach of the six different calibration data files. For example, when thetwo-point interpolation explained with reference to FIG. 12 is to beperformed, the process in the region MS is done by performingextrapolation using calibration data files of the L- and M-regions inthe above method. Instead, it is possible to form an extendedcalibration data file concerning a calibration data file of the M-regionon the basis of an edge portion of this calibration data file, andperform two-point interpolation in the same manner as in the region SI.This method can suppress the generation of an artifact caused by adifference in a portion where these regions connect.

[0083] (Second Method: Substitution)

[0084] In this second method, one calibration data file selected bypredetermined standards from the six different calibration data filesdescribed above is substituted as most appropriate for a calibrationdata file for the set beam thickness. That is, a previously acquiredcalibration data file is directly used without performing anyinterpolation unlike in the above first method.

[0085] For example, if the set beam thickness is present between the M-and S-regions as shown in FIG. 11, a calibration data file of theM-region is selected and used as a calibration data file AA3 for use asshown in FIG. 14. Examples of standards for selecting one of the sixdifferent calibration data files are the following items.

[0086] First, to effectively perform calibration, calibration data filesare required for all detecting elements from which data is acquired byscanning. Therefore, a calibration data file obtained by a beamthickness smaller than when the patient P is scanned cannot be used.That is, in the above example, the data of the M-, L-, and LL-regionsare used without using the calibration data file of the S-region.Second, the scattered ray amount changes in accordance with the beamthickness as described earlier. Hence, data obtained under conditionsclosest to the beam thickness when the patient P is scanned is suitableas a calibration data file to be selected.

[0087] From the foregoing, it is most preferable to use that calibrationdata file pertaining to the smallest beam thickness, which is one ofcalibration data files pertaining to beam thicknesses larger than thebeam thickness when the patient P is scanned. In the example shown inFIG. 14, a calibration data file of the M-region is used.

[0088] Whether to use interpolation of the first method or substitutionof the second method is suitably determined by taking account of theperformance of the X-ray CT scanner 1 or the central control unit 121.Alternatively, the X-ray CT scanner 1 according to the present inventioncan hold both the above two processes such that either process can bepracticed. In this case, an operator of the apparatus can appropriatelyselect a desired process.

[0089] Generally speaking, the interpolation process described aboveenhances the effect of reducing the number of calibration data fileswhich must be acquired in advance in accordance with FIG. 3. On theother hand, the substitution process has an effect of making thisinterpolation process unnecessary.

[0090] (Third Method: Processing Using Calibration Data File EdgePortion)

[0091] This third method is characterized in that a calibration datafile for use is prepared by using an edge portion of a calibration datafile, or by using the method of forming the extended calibration datafile EA by focusing attention on this edge portion, described in thefirst method. In the following explanation, a case in which the set beamthickness is present between the M- and S-regions as shown in FIG. 11 istaken as a representative example. Also, figures to be referred to inthe following explanation are simplified by showing only calibrationdata files of the M- and S-regions.

[0092] First, as a simple method, the extended calibration data filedescribed above can be directly used as a calibration data file for use.That is, as shown in FIG. 15, an extended calibration data file EB basedon an edge portion of a calibration data file of the S-region is formed.In addition, a calibration data file AA4 for use is prepared byconnecting this extended calibration data file EB and the calibrationdata file of the S-region.

[0093] This processing method is basically similar to the concept of thesubstitution process mentioned above. The difference is that acalibration data file of the M-region is used in the substitutionprocess (FIG. 14), but a calibration data file of the S-region isbasically used in this process. This is possible because the extendedcalibration data file EB is formed.

[0094] As a simpler method, an existing calibration data file form(curve form) is directly used without forming any extended calibrationdata file, but caution is exercised on an edge portion. That is, asshown in FIG. 16, a calibration data file in the region MS is shifteddown to meet the output value of a calibration data file of theS-region. A connecting point (≈edge portion) J between the shiftedcalibration data file (of the M-region) in the region MS and thecalibration data file of the S-region is subjected to processing bywhich the two output values continue. This processing has no bigdifference from the concept of the process of obtaining an extendedcalibration data file. Note that as a calibration data file AA4 for use,only the corresponding region need be extracted.

[0095] Still another method is to make the best use of the form of acalibration data file without “shifting” the calibration data file. Forexample, as shown in FIG. 17, a calibration data file (to be referred toas a “connecting calibration data file” hereinafter) CL is preparedwhich connects an edge portion of a calibration data file of theS-region to a calibration data file of the M-region. One end of thisconnecting calibration data file CL is smoothly connected to a portionJ1 near the edge portion of the calibration data file of the S-region.The other end of the connecting calibration data file CL is smoothlyconnected to a portion J2 near the edge portion in a position where thecalibration data file curve of the M-region intersects the boundarydefining the S-region. Note that as a calibration data file AA6 for use,only the corresponding region is extracted similar to FIG. 16.

[0096] The processing method as described above can prevent thegeneration of a discontinuous portion as explained with reference toFIG. 13 in the first method.

[0097] Also, this third method does not use a calibration data fileconcerning a region larger than the set beam thickness, unlike in thesubstitution process of the second method. That is, for a lackingportion (the region MS in FIG. 15 or 17), the extended calibration datafile EB is used (FIG. 15), a calibration data file concerning a regionlarger than the set beam thickness is used after being shifted (FIG.16), or a calibration data file concerning a region larger than the setbeam thickness is used while the connecting calibration data file CL isformed and used (FIG. 17). This makes it possible to use a calibrationdata file of a region smaller than the set beam thickness.

[0098] The use of a calibration data file of a region smaller than thebeam thickness is advantageous when, in the examples shown in FIGS. 15to 17, the beam thickness is close to the size of the S-region and farfrom the size of the M-region. The reason requires no explanation. Inthe above second method, however, a calibration data file of theM-region is prepared as a calibration data file for use even in a caselike this. The significance of this third method is confirmed in thisrespect.

[0099] That is, from this viewpoint it is preferable to use acalibration data file pertaining to a beam thickness closest to the setbeam thickness.

[0100] To make the above operation effective, it is necessary todetermine which of the six different beam thicknesses (=“LL” to “SSS”)corresponding to the six different existing calibration data files theset beam thickness is close to.

[0101] This is done simply by comparing practical numerical values ofthe beam thicknesses, i.e., “LL” to “SSS”, corresponding to the sixdifferent existing calibration data files previously acquired, with apractical numerical value of the set beam thickness. Consequently, it isreadily possible to determine which of “LL” to “SSS” the set beamthickness is close to.

[0102] Alternatively, as shown in FIG. 18, it is also possible toperform processing which conceptually uses a graph in which the outputfrom a certain detecting element of the X-ray detector 112 is plotted onthe ordinate, and the beam thickness is plotted on the abscissa.

[0103] In this graph, the sizes (=beam thicknesses) and the outputs ofthe S- and M-regions are already known by the calibration data fileacquisition process shown in FIG. 3. Also, an appropriate number ofoutputs from the certain detecting element with respect to a beamthickness between the S- and M-regions are acquired beforehand. As aconsequence, the graph shown in FIG. 18 can be formed.

[0104] By referring to the output result obtained by the set beamthickness for the certain detecting element concerning this graph,whether the set beam thickness is close to the S- or M-region isdetermined (see arrows in FIG. 18). On the basis of this result, variousprocesses explained in this third method are performed if the set beamthickness is close to the S-region, and the substitution processexplained in the second method is performed if the set beam thickness isclose to the M-region. That is, processing using a calibration data fileconcerning a beam thickness close to the set beam thickness can beperformed.

[0105] This processing using FIG. 18 is advantageous because, as shownin FIG. 18, the outputs of a plurality of regions do not strictly have aproportional relationship in some instances. That is, if the beamthickness and the output have a nonlinear relationship as shown in FIG.18, it is difficult for the simple comparison described above toaccurately determine which region the set beam thickness is close to.However, the processing herein mentioned makes this possible. Note thatthe graph shown in FIG. 18 is formed for a “certain detecting element”.However, the present invention is not limited to this example. Forexample, a graph as shown in FIG. 18 can also be formed for “severaldetecting elements (specific detecting elements) selected with highsymmetry from the X-ray detector 112”, or for a “plurality of detectingelements (specific detecting elements) in the same channel”.

[0106] In this third embodiment, the set beam thickness is presentbetween the M- and S-regions. However, other cases (e.g., a case inwhich the set beam thickness is present between the L- and M-regions)can also be exactly similarly processed.

[0107] In step T5 of FIG. 9, the calibration data files AA1 to AA6 foruse acquired by the first to third methods as described above aresubjected to actual correction for scan data. It is obvious that thiscorrection process using the calibration data files AA1 to AA6 for usecan appropriately correct the sensitivity of the X-ray detector 112.

[0108] (Fourth Method: Scattered Ray Correction)

[0109] This fourth method is characterized in that variously settablebeam thicknesses can be corrected by applying a scattered ray correctionprocess. As described previously, scattered rays are excessivelydetected X-ray components other than direct X rays. The larger the beamthickness and the larger the diameter of the patient P, the larger theamount of scattered rays (FIGS. 7A, 7B, 8A, and 8B). “Scattered raycorrection” is the process of excluding such scattered rays fromprojection data, and obtaining projection data consisting substantiallyprimarily of direct X rays. This scattered ray correction process can beperformed by using, e.g., the preprocessing unit 123, the centralcontrol unit 121, or a dedicated arithmetic unit (to be referred to as asecond correcting means hereinafter). As the scattered ray correctionprocess of this fourth method, a method disclosed in, e.g., Jpn. Pat.No. 1631264 or Jpn. Pat. Appln. KOKAI Publication No. 11-89827 can beused.

[0110] The scattered ray correction process disclosed in Jpn. Pat.Appln. KOKOKU Publication No. 1631264 will be briefly described below.That is, as shown in FIG. 19, an X-ray diagnostic apparatus of thispublication includes an X-ray shielding means XS. This X-ray shieldingmeans XS is constructed by arranging X-ray shielding members such aslead pieces XSB at equal intervals on an X-ray transmitting member XSAobtained by shaping, e.g., acrylic resin into the form of a thin plate.This X-ray shielding means XS can move as indicated by arrows shown inFIG. 19, and can make X-rays emitted from an X-ray tube 111 shielded orunshielded with respect to an X-ray detector 112. Note that X-rays are,of course, shielded or unshielded at the positions of the lead piecesXSB.

[0111] When this X-ray shielding means XS is positioned in the field ofirradiation, the X-ray diagnostic apparatus can acquire X-ray shieldeddata. During the acquisition of this X-ray shielded data, no X-rays aredirectly incident on detecting elements corresponding to the positionsof the lead pieces XSB. So, the resulting output (scattered ray data)reflects the presence of scattered rays. After that, therefore, on thebasis of the relationship between the positions of the lead pieces XSBand the scattered ray data corresponding to the individual positions,the distribution (scattered ray intensity data) of scattered rayintensity on the entire surface (=all detecting elements) of the X-raydetector 112 can be calculated. According to the publication, this isdone by data interpolation using a sampling function.

[0112] By calculating a difference between the scattered ray intensitydata thus obtained and original image data obtained by positioning theX-ray shielding means XS outside the irradiation field, projection dataexcluding the influence of scattered rays is obtained.

[0113] The scattered ray correction process disclosed in Jpn. Pat.Appln. KOKAI Publication No. 11-89827 is substantially as follows. Thatis, as shown in FIG. 20, an X-ray CT scanner of this publicationincludes a channel-direction collimator CDC and a slice-directioncollimator SDC on the front surface of an X-ray detector 112. Thechannel-direction collimator CDC prevents scattered rays in a channeldirection from entering the X-ray detector 112. The slice-directioncollimator SDC prevents scattered rays in a slice direction (parallel tothe axial direction of the patient P) from entering the X-ray detector112. Collimator plates CDC1 are densely arranged in thechannel-direction collimator CDC. Collimator plates SDC1 are “sparsely”arranged in the slice-direction collimator SDC.

[0114] In this X-ray CT scanner, as shown in FIGS. 21A and 21B, theaction of the slice-direction collimator SDC (physically) removesscattered rays only in the positions of the collimator plates SDC1.Consequently, detecting elements A to D of the X-ray detector 112immediately below these positions detect only direct X rays (see “densehatched portions” in FIGS. 21A and 21B).

[0115] Detecting elements X2 and X3, X6 and X7, X10 and X11, and X14 andX15 on the two sides of the detecting elements A, B, C, and D,respectively, detect X rays from which scattered rays are slightlyremoved but which still contain remaining scattered ray components.Remaining detecting elements X1, X4, X5, X8, X9, X12, X13, and X16detect X rays (direct X rays +scattered rays) from which no scatteredray components are removed at all (in FIGS. 21A and 21B, “broken-lineportions” indicate physically removed scattered rays, and “sparsehatched portions” indicate detected scattered rays).

[0116] In the above publication, from the differences between thesemodes, the distribution of direct ray components is estimated on thebasis of the outputs from the detecting elements A to D, and thedistribution of direct ray components and scattered ray components isestimated on the basis of the outputs from the detecting elements X1,X4, X5, X8, X9, X12, X13, and X16 (FIGS. 21A and 21B). By subtractingthe former from the latter, the distribution of only scattered rays onthe front surface of the collimator can be obtained. By multiplying thisscattered ray distribution by a previously calculated removal ratio, ascattered ray amount incident on each detecting element can beestimated.

[0117] By subtracting the thus estimated scattered ray amount fromactual scan data, projection data from which the influence of scatteredrays is eliminated is obtained. The “removal ratio” is the ratio of thescattered ray amount removed when the slice-direction collimator SDC ispresent to the total scattered ray amount when this collimator SDC isabsent.

[0118] In this fourth method, the above scattered ray correction isfirst performed in the calibration data file acquisition processexplained with reference to FIG. 3. Note that this calibration data fileacquisition is performed only for a calibration data file concerning themaximum beam thickness determined by the size of the X-ray detector 112.That is, in the example shown in FIG. 4, only a calibration data fileconcerning the LL-region is acquired. Therefore, in the calibration datafile acquisition process shown in FIG. 3, the processing in step S5 isomitted.

[0119] The actual scan conditions, however, include not only the beamthickness but also conditions such as the tube voltage of the X-ray tube111. Hence, although “only a calibration data file pertaining to theLL-region is acquired”, a necessary number of calibration data filesmust be acquired for the other parameters. That is, it is necessary toacquire, e.g., a “calibration data file for a tube voltage v [V] of theX-ray tube 111 when the beam thickness is the LL-region”, and a“calibration data file for a diameter d [m] of the patient P when thebeam thickness is the LL-region”. In this example, however, as alreadymentioned above, only the beam thickness is taken into consideration asthe scan condition.

[0120] The timing at which scattered ray correction is performed for theacquired calibration data file of the LL-region is between steps S3 andS4 in FIG. 3. In this way, the scattered-ray-corrected calibration datafile is stored (step S4 in FIG. 3).

[0121] Subsequently, the actual patient scanning process explained withreference to FIG. 9 begins. As described previously, a beam thickness isfreely set at a fine pitch (steps T1 and T3 in FIG. 9), and data of aminimum necessary region of the patient P is acquired. A calibrationdata file prepared in step T2 of FIG. 9, i.e., a calibration data filefor use in this fourth method, is naturally the scattered-ray-correctedcalibration data file concerning the LL-region.

[0122] In step T5 of FIG. 9, the preprocessing unit 123 performs variouscorrecting processes and scattered ray correction for the acquiredpatient scan data. Subsequently, the sensitivity of the X-ray detector112 is corrected by using the scattered-ray-corrected data file(calibration data file for use) related to the LL-region on thisscattered-ray-corrected scan data.

[0123] In this processing, sensitivity correction is performed by usingthe data (the scattered-ray-corrected calibration data file and scandata) from which the influence of scattered rays is eliminated byscattered ray correction. This eliminates the problem of the differencebetween scattered ray amounts produced by the difference between thebeam thickness when calibration data files are acquired and that whenthe patient is scanned. As a consequence, a highly accurate image havinglittle artifact is obtained.

[0124] In the above forth method, outlines of Jpn. Pat. No. 1631264 andJpn. Pat. Appln. KOKAI Publication No. 11-89827 are explained asscattered ray correction processes. In the present invention, however,it is basically possible to use scattered ray correction based on anymethods in addition to the above two scattered ray correction processes.In any case, the function and effect described above are achieved.

[0125] (Fifth Method: Combined Use of Scattered Ray Correction andCorrection Process Based on Several Different Calibration Data Files)

[0126] This fifth method is characterized by combining the interpolationprocess, the substitution process, and the processing using acalibration data file edge portion described in the first, second, andthird methods, with the scattered ray correction process described inthe fourth method. In the following description, the combination of theinterpolation process of the first method and the scattered raycorrection process will be explained.

[0127] In this fifth method, similar to the first method describedabove, a plurality of different calibration data files concerning apredetermined beam thickness are acquired. Each of these calibrationdata files is subjected to scattered ray correction in the same manneras in the fourth method, and stored (the processing shown in FIG. 3including scattered ray correction is performed).

[0128] The procedure shown in FIG. 9 then starts. In accordance with thescan condition (i.e., the “beam thickness” in this method) of thepatient, the scattered-ray-corrected calibration data files areinterpolated to estimate a calibration data file for use of patient scandata (see step T2 in FIG. 9 and the description in the first method).Scattered ray correction is also performed for the patient scan data asin the fourth embodiment. Sensitivity correction is performed for thethus obtained scattered-ray-corrected scan data by using the“calibration data file for use” based on the several differentscattered-ray-corrected calibration data files estimated above (step T5in FIG. 9).

[0129] This processing can simplify the scattered ray correctionprocess. That is, in this fifth method, the interpolation process in thefirst method and the scattered ray correction process in the fourthmethod, both of which are proven to be effective in appropriatelycorrecting a variously settable beam thickness, are performed incombination. This can relatively alleviate the duties to be fulfilled bythe scattered ray correction process. This simplification of thescattered ray correction process is sometimes necessary depending on,e.g., the scheme of scattered ray correction and the weight, time, andthe like of the correction process.

[0130] Also, in the above fifth method, a highly accurate image havinglittle artifact is obtained for the same reason as above, even when theaccuracy of scattered ray correction is low.

[0131] As has been described above, when the various processes explainedas the first to fifth methods are performed, an appropriate calibrationdata file for use for a variously settable beam thickness can beobtained only by acquiring one or several different calibration datafiles. Basically, therefore, accurate sensitivity correction can beperformed whatever the beam thickness is set.

[0132] Accordingly, in these methods, usable beam thicknesses are notlimited unlike in conventional methods, so the beam thickness can befreely set. As a consequence, the patient P is not unnecessarily exposedto X rays.

[0133] In the first to third methods and the fifth method, six differentcalibration data files from “LL” to “SSS” are prepared for predeterminedbeam thicknesses. However, the present invention is not restricted tothis form. Basically, any number of different calibration data files canbe prepared. Also, in the methods except for the first method in whichinterpolation is performed, the processing can be performed in principleonly by acquiring a calibration data file for “one” beam thickness.

[0134] Generally speaking, however, the first method can infinitelyperform the processing for any beam thickness in principle, becauseinterpolation is performed. Therefore, the number of calibration datafiles to be prepared can be small. In the second and third methods,however, it is preferable to prepare a larger number of calibration datafiles than in the first method.

[0135] The present invention is most suitably applicable to a so-calledcone-beam X-ray CT scanner. However, it is of course also possible toapply the present invention to a “multi-slice X-ray CT scanner”described in “2 Description of the Related Art”.

[0136] Furthermore, it is favorable to apply the following modificationto the form of practicing the “scattered ray correction processes”described in the fourth and fifth methods.

[0137] (Determination of Propriety of Scattered Ray Correction Process)

[0138] This scattered ray correction process is characterized in thatwhether to perform scattered ray correction, or the amount or intensity(intensity of the degree of correction) of scattered ray components tobe actually subtracted, is determined in accordance with a differencebetween set beam thicknesses and the like, for the fourth and fifthmethods using scattered ray correction.

[0139] As already described several times, the scattered ray amountstrongly depends upon the scan conditions, particularly the beamthickness and the diameter of the patient P; the larger the beamthickness and the larger the diameter of the patient P, the larger thescattered ray amount (FIGS. 7A, 7B, 8A, and 8B).

[0140] Conversely speaking, the influence of scattered rays is not solarge if the beam thickness or the patient size (equivalent to theimaging region of an axial section as a scan condition) is small.

[0141] Accordingly, when the beam thickness or the patient size is usedas a parameter, it is possible to determine whether to perform scatteredray correction, or to determine the amount or intensity of scattered raycomponents to be actually subtracted. More specifically, if the beamthickness or the patient size is large, the scattered ray amountincreases, so scattered ray correction is performed or the amount ofintensity of scattered ray components to be actually subtracted isincreased. In contrast, if the beam thickness or the patient size issmall, the scattered ray amount reduces, so no scattered ray correctionis performed or the amount or intensity of scattered ray components tobe actually subtracted is decreased.

[0142] The “amount or intensity of scattered ray components to beactually subtracted” described above can be determined by multiplyingso-called “raw” scattered ray components (“scattered ray intensity data”in Jpn. Pat. No. 1631264, and an “estimated scattered ray amount”calculated by removal ratio multiplication in Jpn. Pat. Appln. KOKAIPublication No. 11-89827), purely calculated or estimated, by anappropriate proportional coefficient a. As is evident from the aboveexplanation, the proportional coefficient a can be 0<a<1 or a≧1.

[0143] This processing can prevent the occurrence of abuses whenscattered ray correction is performed although the necessity of theprocess is weak. “Abuses” herein mentioned simply include a prolongedoperation time caused by the scattered ray correction process, and alsomean a case as shown in FIG. 22.

[0144] That is, when X-ray intensity as indicated by the broken line inFIG. 22 is detected, net X-ray data P1 can be obtained even if scatteredray components SDM extending at the bottom of this X-ray intensity areremoved. However, when this X-ray intensity is as indicated by the solidline in FIG. 22, the amount of (the degree of contribution of) scatteredrays is large relative to net X-ray data P2. Therefore, if the scatteredray components SDM are removed in this state, the net X-ray data P2becomes almost “0”, and this makes it difficult, or impossible, toobtain the value of the data. This scattered ray correction process caneliminate such “abuses”.

[0145] As described above, whether to perform the scattered raycorrection process, or the amount of intensity of scattered raycomponents to be actually subtracted, is determined by using theabove-mentioned parameter. This determination is preferably performedsuch that the degree of contribution of scattered ray components withrespect to the whole X-ray data detected is about 5 to 10%.

[0146] In the above description, whether to perform the scattered raycorrection process is determined on the basis of the beam thickness “or”the patient size. However, the present invention is not limited to thisform. For example, it is also possible to regard the beam thickness andthe patient size as having an organic relationship. In this case, noscattered ray correction process is performed as long as both the beamthickness and the patient size are equal to or smaller than first andsecond predetermined values (these values have the nature of a watershedwhich determines whether to perform the scattered ray correctionprocess). That is, in this processing, the scattered ray correctionprocess is performed if the patient size is larger than the secondpredetermined value although the beam thickness is equal to or smallerthan the first predetermined value.

[0147] In short, the present invention can determine whether to performthe scattered ray correction process, or to determine the amount ofintensity of scattered ray components to be actually subtracted, inaccordance with the combination of the beam thickness and the patientsize.

[0148] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit and scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. An X-ray CT scanner for reconstructing a CT imageof an object to be examined, based on projection data obtained byscanning the object using an X-ray source for radiating an X-ray beamand an X-ray detector for detecting an X-ray transmitted through theobject, comprising: a setting unit configured to arbitrarily set athickness of the X-ray beam with respect to a slice direction of theobject.
 2. A scanner according to claim 1, further comprising adisplaying unit configured to display information about the thickness ofthe X-ray beam.
 3. A scanner according to claim 2, wherein saiddisplaying unit displays lines pertaining to the thickness of the X-raybeam, and wherein the scanner further comprises an input deviceconfigured to manipulate the lines by an operator.
 4. A scanneraccording to claim 3, wherein the lines show ends of the X-ray beam withrespect to a slice thickness.
 5. A scanner according to claim 3, whereinthe lines show a center and ends of the X-ray beam with respect to aslice thickness.
 6. A scanner according to claim 1, further comprising adisplaying unit configured to display at least one of numerical valuesand cursors on a scanogram to support an inputting of a desiredthickness of the X-ray beam.